Applications of magnetic bearings in rotary machines have developed considerably in the last few years because of the enormous advantage obtained by operating them directly in the process gas of the machine in question, without sealing. Thus, and in non-limiting manner, magnetic bearings are to be found on turbo-expanders, on refrigeration compressors, on electric motors for driving compressors, etc.
In ordinary applications, magnetic circuits are all based on silicon iron. The ferromagnetic sheets forming the laminated magnetic material of such circuits thus have the advantage of presenting magnetic characteristics that are well defined and guaranteed by their suppliers, in particular a hysteresis cycle that is limited and magnetic permeability and saturation induction that are high.
Nevertheless, for more particular applications, in a medium that is acid, corrosive, or carrying particles, direct contact between ferromagnetic laminations as used in windings and process gases is found to be impossible without installing jackets that isolate the magnetic circuits of the stator from the aggressive environment, and thus enable conventional materials based on silicon iron to be used for those circuits.
Furthermore, such a jacket conventionally needs to have a coefficient of thermal expansion that is identical to that of the ferromagnetic laminations supporting the winding so that it does not deform as a function of temperature, which would quickly lead to the stator being destroyed by coming into contact with the rotor.
Thus, a jacketed magnetic bearing is commonly made up of an assembly of wound ferromagnetic laminations inserted in an enclosure that is hermetically sealed against the environment in which it is used in order to protect it from the corrosive attacks by said environment. Each of the components of the enclosure, and also the connections between them, must be capable of withstanding the corrosion that can be caused by that environment. Furthermore, in so-called “oil and gas” environments, with more specific applications coming under the standard ANSI/NACE MR0175/ISO 15156 “Petroleum and natural gas industries—Materials for use in H2S-containing environments in oil and gas production”, hardness constraints put a limit on the materials that can be selected.
Conventionally, the magnetic material generally used for the jacket is a martensitic stainless steel with precipitation hardening. Unfortunately, in order to comply with the recommendations of the above-mentioned standard, welding such a material requires heat treatment (typically high temperature annealing performed at about 620° C.), which is incompatible with a jacket that is fine or with the high-temperature capacity of the wound ferromagnetic laminations themselves, which conventionally cannot withstand temperatures higher than 250° C. Without such heat treatment, welds lose their anti-corrosion compatibility, thereby greatly complicating fabrication of the bearing and very significantly increasing the costs of using it.
In order to recover such compatibility, U.S. Pat. No. 7,847,454 proposes using a bearing jacket made of two materials that is subjected to heat treatment before final machining, with only a central portion thereof being made of martensitic stainless steel with precipitation hardening, the remaining portions of the jacket being made of a non-magnetic material such that during final assembly operations, the only welds that remain to be made are all welds that are compatible with the standard NACE MR0175/ISO 15156, such as welds between non-magnetic materials. Likewise, it is necessary to add numerous inserts of non-magnetic material on other portions of the support, with those parts being welded to the magnetic portions, and with the welds being subjected to heat treatment, and then being re-machined in order to give them their final shapes prior to final assembly.
FIG. 3 shows an example of an active magnetic bearing stator arranged in a hermetically sealed jacket, as described in the above-mentioned patent. The enclosure is commonly constituted by a cylindrical support 10 having a longitudinal axis 12 with a set of wound ferromagnetic laminations 14 interference fitted therein. The support is provided with walls 16, 18 on each of its side faces, and the enclosure is finished off by adding a cylindrical jacket 20 on its inside diameter in contact with the process gas. The enclosure is completely filled with a potting resin 22 thereby reinforcing its mechanical strength so that it can be placed in pressurized surroundings (up to a few hundreds of bars). The windings (electromagnet coils) of the bearing stator are connected through a conventional hermetically sealed socket (not shown) to electronic control circuits 24 that, as shown, may be located outside the hermetic enclosure as created in this way.
The jacket 20 is constituted by a cylindrical central portion 20A made of a magnetic material, typically a martensitic stainless steel with precipitation hardening of the 17-4 PH type, having extensions 20B fitted at both ends that are made of a non-magnetic material, typically of Inconel. The walls 16, 18 are also made in two portions with inserts 16A, 18A previously welded to the bearing support 10, heat-treated, and then re-machined to their final dimensions, and cheek-plates 16B, 18B making the connections between these inserts and the jacket 20, and more precisely its extensions 20B. These inserts and cheek-plates are made of a non-magnetic material, typically likewise of Inconel. The support 10 of the bearing is conventionally made of 17-4 PH in order to have a coefficient of thermal expansion that is practically identical to that of the wound assembly 14 based on silicon iron, as is necessary to guarantee that the ferromagnetic laminations remain an interference fit within the support up to high ambient temperatures (typically up to 200° C.)
This stator is assembled as follows. The extensions 20B are initially welded to the central portion 20A in order to form the jacket 20, and then the assembly is heat-treated at around 620° C., and is finally reworked by machining in order to mitigate the deformations created by the treatment at high temperature. Likewise, the side inserts 16A, 18A are welded to the bearing support 10, and then the assembly is also subjected to heat treatment at about 620° C. Naturally the heat treatments have the effect of producing geometrical deformations that are subsequently reworked by machining. The wound ferromagnetic laminations of the bearing 14 can then be inserted as an interference fit inside the support 10, after which the cheek-plates 16B, 18B are welded to the inserts 16A, 18A, and the jacket 20 is welded to the cheek-plates, with none of these welds requiring subsequent heat treatment.
Thus, by providing the jacket and also the support with non-magnetic inserts, it becomes possible, once these inserts have been welded both to the jacket and to the support, and once the assembly has been subjected to high-temperature annealing, to place the wound ferromagnetic laminations inside the enclosure and then to weld the non-magnetic inserts together, without having recourse to any particular heat treatment, and taking action at temperatures that are compatible with the materials of the laminations and of the windings.
Unfortunately, the initial heat treatment operations, and the necessary reworking are lengthy and complex, and therefore particularly expensive. They are also sources of major fabrication anomalies, in particular in the two-material bearing jacket, which is particularly difficult to connect with the inserts because of its very small thickness.